Basically: the language specification. But actually, Type.FullName uses the BCL definitions, not the C# definitions - and interestingly they disagree. For example:

namespace X {
public class Y {
public class Z {}
}
}

To C#, Z is X.Y.Z; to the BCL it is X.Y+Z. The representation of generics changes too - with the BCL using back-ticks and numbers rather than angular brackets. I believe the BCL uses the CLI's format of types (which has a separate specification), but if you think about it: it is not required to do so (except for during reflection-emit).

AFAIK, these separators are not exposed via anything like Path.PathSeparator - but is, as you say, hard coded into the Type etc classes.